First, let's consider a different & simpler problem, involving eggs
Suppose you had 28 eggs. How many dozen eggs do you have? 2.
28 // 12 gets you 2 dozen
How many left over (that didn't fit in the 2 cartons of 12)? 4 eggs.
28 % 12 gets you "4 left over"
The pennies/shillings/pounds is a little more complex, since you need to apply this kind of logic twice, but the basic idea is similar!